"Job ter Burg" posted this in 2014 - maybe it will help you as what he says is correct:
SubCap supports two formats. One is .STL, the other is .TXT in a specific format, but it can be created or modified by any text editor. It's basically text that looks like this:

@ This file written with the Avid Caption plugin, version 1

01:00:49:21 01:00:54:15
Text line one.

02:56:53:00 02:56:55:13
Text line two.
And three.

As said, you'll need the timecodes, otherwise SubCap has nothing to time the captions to.